The present invention relates to an interferometer arrangement, particularly to a transistor arrangement such as a resonant tunneling transistor arrangement with superconducting gate electrodes. The invention also relates to a logical element comprising such a resonant tunneling transistor. Still further the invention relates to a method of controlling the conductance of a tunneling superconductor arrangement.
Resonant tunneling transistor devices are known which consist of a normal metal source electrode and a normal metal drain electrode which are connected through tunnel barriers to a normal metal island forming a base electrode. However, even if the output conductance, G, of such devices can be varied, it can not be varied or increased to a sufficient extent for a number of applications. In such devices the output conductance can be varied through varying the supercurrent in the superconducting gate electrodes used to control the source-drain current. Thus, what is needed to change the conductance is the provision of a variation in this supercurrent to vary the quantum mechanical phase difference xcex94xcfx86 between the two superconducting gate electrodes. It is a problem that such devices only find a limited practical use and they probably would not find an extended application as logical elements since they merely can be used as OR-elements but also as such they can hardly be used. Such a device is described by E. Toyoda and H. Takayanagai, in Proceedings of 12th International Conference on the Electronic Properties of Two-Dimensional Systems, EP2DS, Tokyo 1997. Furthermore another such device is described in Phys. Rev. Lett. Vol. 74, p 5268 (1995) by V. T. Petrashov, V. N. Antonov, P. Delsing and T. Claeson which herewith is incorporated herein by reference.
What is needed is therefore a transistor arrangement as referred to above, the output conductance of which can be made considerably higher than in hitherto known arrangements and which enables a larger variation in conductance than known arrangements. Particularly an arrangement is needed through which the output conductance can be controlled in an easy and flexible manner. Still further a transistor arrangement is needed which is efficient, easy to fabricate, reliable and which can produce a most pronounced output signal. Still further a method of controlling the conductance of a tunneling superconducting arrangement, e.g. a transistor is needed which fulfills the above mentioned objects.
Therefore a transistor arrangement is provided which comprises a source electrode and a drain electrode, a base electrode to which said source and drain electrodes are connected through tunnel barriers so that the base electrode forms a double barrier quantum well. First and second superconducting gate electrode means are provided to control the source-drain current ISD. The base electrode comprises a ferromagnetic material which enables resonant tunneling of source-drain electrons when there are bound states within the quantum well matching the energy of the source-drain electrons. Otherwise the double barrier quantum well prevents the source-drain current, i.e. the transport of source-drain electrons. However, if there are such bound states within the quantum well matching the energy of the source-drain electrons, they will be able to pass through the ferromagnetic base electrode, particularly a mesoscopic ferromagnetic island, by resonant tunneling. The bound states in the quantum well structure are also called Andreev levels which can be said to be provided by and controlled by the superconducting gate electrode means.
According to the invention the output conductance (G) depends on a first and a second parameter and first and second controlling means are provided to control the output conductance through controlling said parameters. The first parameter is the phase difference xcex94xcfx86 between the first and the second superconducting electrodes and first controlling means are provided enabling the control of said phase difference. Said first controlling means can be provided in different ways. According to one advantageous implementation the first controlling means comprises means for changing the super-current, IG, provided by a control current source supplied between the two superconducting gate electrodes.
In an alternative embodiment the first controlling means comprises means connecting the first and the second superconducting gate electrodes so as to form a loop and means for application of a magnetic field to said loop.
The second parameter on which the output conductance depends is the exchange interaction potential (energy) of the ferromagnetic base electrode. The second controlling means advantageously comprises a voltage source and a voltage (VB) is applied to said ferromagnetic base electrode. The output conductance, G, according to the invention is given by xcex94ISD/xcex94VB and thus is strongly dependent on the quantum mechanical phase difference xcex94xcfx86 between two superconductors and of VB. In order to provide a particularly high conductance, or particularly an output signal according to a particular implementation, the phase difference xcex94xcfx86 is controlled to be substantially (2n+1)xcfx86, wherein n=0, xc2x11, . . . Advantageously the applied voltage VB should substantially correspond to the interaction energy 2hO/e of the ferromagnetic base electrode. If xcex94xcfx86 is not equal to an odd multiple of xcfx80, there will be substantially no resonant tunneling and the conductance, which is proportional to the (transparency)2, i.e. the transport of source-drain electrodes, will be low for any applied voltage VB. If however VB is substantially equal to, or close to, 2hO/e, a variation in the supercurrent IG through the gate electrode will result in a variation in the output conductance over a wide range. This is due to the fact that the Andreev levels then will be concentrated near the exchange energy hO, thus considerably increasing the resonant tunneling. Thus, if the applied voltage VB is decreased to a value well below 2hO/e, the macroscopic resonant transmission of electrons will be switched off as well as the sensitivity of the conductance G to variations in the phase difference. This means that a very high conductance, or an output signal, will be observed, according to the present invention, only when the first and the second parameters both fulfill the given requirements at the same time or, in other words, when IG (a supercurrent produced in an appropriate manner) and VB both are present.
Maximum output conductance is particularly provided for xcex94xcfx86=2(n+1)xc3x97xcfx80 and VB=2hO/e. In an advantageous embodiment the source and the drain electrodes comprise a normal metal such as Au, Ag, Bi. The superconducting gate electrodes are advantageously made of Pb, Al, Nb, Yt, Ba, CuO but also other alternatives are possible. High temperature superconducting (high-TC) materials can be used as well as non-high-TC-materials. In advantageous implementations the ferromagnetic base electrode consists of La, Sr, Ca, MnO but also other ferromagnetic materials are possible.
In a particular implementation the arrangement forms a logical AND-element, an output signal being provided only when xcex94xcfx86≈2(n+1)xc3x97xcfx80 and VB≈2hO/e, i.e. when both VB and IS are input or activated.
In a particular implementation the ferromagnetic base electrode is mesoscopic.
According to the invention a logical element may also be provided which comprises a resonant tunneling transistor comprising a source electrode and a drain electrode, a base electrode to which said source and drain electrodes respectively are connected via tunnel barriers and first and second superconducting gate electrodes arranged to control the source-drain current ISD. The base electrode comprises a ferromagnetic material, particularly an island of a mesoscopic ferromagnetic material, and to provide an output signal corresponding to a high conductance, a first input signal is provided to produce a phase difference between the superconducting gate electrodes being (2n+1)xcfx80 and a second input signal in the form of a voltage VB is applied to the ferromagnetic base electrode, which voltage approximately corresponds to 2hO/e which is the interaction exchange energy of the ferromagnetic base electrode.
Particularly the output signal corresponding to the output conductance, which should be high, depends on the first and the second parameter respectively and first and second controlling means are provided to control the output conductance wherein both said first and second controlling means have to be activated to provide an output signal. The first input signal is particularly provided by first controlling means comprising means for changing the supercurrent and second controlling means comprising a voltage source provides the voltage VB. In a particular implementation the second controlling means comprises means connecting the first and the second superconducting gate electrodes to provide a loop, or they are formed as loops, and means for applying a magnetic field to said loop resulting in the phase difference being an odd multiple of xcfx80. Alternatively said first controlling means comprises means for changing the super current such as a control current source thus affecting the phase. The second controlling means particularly comprises a voltage source to provide the voltage VB.
According to the invention a circuit is also provided which comprises a number of logical elements of which at least some of the AND elements comprise logical elements according to any one of claims 15-18.
Particularly a method of controlling the conductance of a resonant tunneling superconductor arrangement, e.g. a transistor is also provided. The transistor comprises a source and a drain electrode, a base electrode to which the source electrode and the drain electrode respectively are connected through tunnel barriers, the base electrode forming a double barrier quantum well, and first and second superconducting gate electrodes controlling the source-drain current. The method includes the steps of:
controlling the phase difference between the superconducting gate electrodes to make it assume such a value that resonant tunneling is enabled,
applying of voltage to the base electrode which enables transportation of electrons through the provision of Andreev levels, i.e. bound states in the quantum well. Particularly the method comprises the steps of applying a voltage close to 2hO/e corresponding to the interaction exchange energy of the base electrode which comprises a ferromagnetic material, and providing a phase difference between the superconducting gate electrodes which corresponds to (2n+1)xcfx80, wherein n=0, xc2x11, . . . The provision of a phase difference which is an odd multiple of xcfx80 can be done in different ways, such as interconnecting the superconducting gate electrodes to form a loop, and applying a magnetic field through said superconducting loop. Alternatively the phase difference can be controlled through changing the current IG using for example a control current source thus producing a supercurrent IS flowing through the superconductors..